Control valves for vehicle hydraulic systems

ABSTRACT

A control valve assembly incorporates a piston and a flow control valve which are both biassed in the same direction against pressure in an hydraulic accumulator by first and second respective springs. Accumulator pressure in a control chamber also acts to urge the flow control valve in the same direction as it is biassed by the second spring and pressure from a pump acts to bias the flow control valve in the opposite direction. A pilot dump valve controlling communication between the chamber and an outlet connection is normally biassed into a closed position by a third spring. The dump valve is adapted to open at a cut-out pressure to dump pressure from the chamber, and the cut-out pressure is determined by the area of the seating of the dump valve over which the pressure in the chamber acts, and the force in the third spring. A cut-in pressure at which the dump valve closes is determined by the area of the piston and the force in the first spring.

This application is a division of application Ser. No. 635,475, filedJuly 30, 1984, now Pat. No. 4,503,876, which is a continuation ofapplication Ser. No. 355,343, filed Mar. 8, 1982, now abandoned.

This invention relates to control valve assemblies for vehicle hydraulicsystems of the kind for limiting the pressure to which an hydraulicaccumulator for supplying services can be charged from a high pressurepump.

Valve assemblies of this kind may comprise flow dividers arranged todivide the flow from the pump between steering gear connected to anoutlet connection, and the accumulator, or unloader valves in which theoutlet connection is simply connected to a tank for hydraulic fluid.Hereinafter such flow dividers or unloader valves will be referred to avalves "of the kind set forth".

Some known valve assemblies of the kind set forth incorporate valvemeans which senses pressure in the accumulator, requiring the provisionof an elastomeric seal or a high precision ground spool which exhibitpoor hysterisis at low temperatures.

In another known valve assembly of the kind set forth pilot valve meanssenses pump pressure and, when subjected to a predetermined pumppressure, is operative to dump such pump pressure to permit a one-wayvalve between the pump and the accumulator to close, thereby preventingany further increase in the pressure in the accumulator, with theaccumulator pressure acting to open a further valve to divert flow fromthe pump to the tank.

According to our invention a control valve assembly of the kind setforth comprises a piston, a first spring biassing the piston in onedirection in opposition to accumulator pressure, a flow control valveadapted to co-operate with the piston, a seacond spring biassing theflow control valve in the same direction as that in which the piston isbiassed by the first spring, accumulator pressure in a control chamberacting on the flow control valve in the same direction as the secondspring, and pump pressure acting on the flow control valve in theopposite direction, a pilot dump valve for controlling communicationbetween the portion of the flow control valve which is subjected toaccumulator pressure and an outlet connection, and a third spring forbiassing the dump valve into a closed position to isolate the controlchamber from the outlet connection, movement of the dump valve into anopen position at a predetermined cut-out pressure dumping pressure fromthe control chamber to permit the flow control valve to be urged into anunloading position due to the pressure in the accumulator acting on theflow control valve through the piston and against the loading in thefirst spring and the second spring, reduction in accumulator pressuretaking place until the force exerted on the piston by the first springenables the piston to move in the opposite direction against thepressure at which the flow control valve again establishes communicationbetween the pump and the accumulator.

It follows therefore that the cut-out pressure is determined by the areaof the seating of the dump valve over which pressure in the controlchamber acts and the force in the third spring, and the area of thepiston and the force in the first spring determine the cut-in pressure.

By changing the value of the springs the cut-in/cut-out range can easilybe changed, particularly since we can arrange to make an externaladjustment in the load applied by the third spring.

When the valve assembly comprises a flow divider the outlet connectionis connected to steering gear and flow through the flow control valve isdetermined by a restriction so that a substantial proportion of the flowfrom the pump is always supplied to the outlet connection.

When the valve assembly comprises an unloader valve no restriction isprovided in the flow control valve and the flow control valve may beintegral with the piston. The outlet connection is connected to thereservoir.

One embodiment of our invention is illustrated in the single FIGURE ofthe accompanying drawings which is a longitudinal section through aunloader valve.

The unloader valve illustrated in the drawing comprises a housing 1provided with a longitudinally extending stepped bore 2 having a firstthreaded portion 3 of greatest diameter for connection to an hydraulicaccumulator 4, a second portion 5 of intermediate diameter, a thirdportion 6 of a smaller diameter accommodating a seal 7 through whichworks a piston 8, and a further portion 9 of smallest diameter in whichworks a spool 10 defining a flow control valve, the outer end of thatportion being closed by a plug 11.

A first strong spring 12 acts between a retainer 13 for the seal 7 and ahead 14 at the end of the piston 8 which is remote from the spool 10 inorder to urge the head towards a stop member 15 which is clamped by theaccumulator 4 against a shoulder 16 at a step in diameter between thebore portions 3 and 5.

A radial port 17 and an axially extending passage 18 in the piston 8provide communication between the accumulator 4 and the bore portion 5which, in turn, is connected to services, for example a brake-actuatingbooster B or hydraulic suspension struts, through an inclined passage19.

The spool 10 is urged by a second spring 20 into engagement with theadjacent end of the piston 8. The spool 10 is provided with a radialpassage 21 which provides communication between a connection 22 to anhydraulic pump P and a communicating longitudinal passage 23 in thespool 10. The passage 23 leads to a control chamber 24 defined in thebore portion 9 between the plug 11 and the adjacent end of the spool 10and in which the spring 20 is housed. The control chamber 24 leads tothe passage 19 through a spring loaded one-way valve 25 and to a pilotdump valve 26.

The dump valve 26 comprises a valve member 27 of substantial diameterwhich is urged by a third spring 28 into engagement with a seating 29 ofrelatively small area and which is exposed to the control chamber 24.The valve member 27 has a skirt 30 which, when the head is in engagementwith the seating 29, closes a passage 31. The passage 31 is connected toan outlet connection 32 leading from the bore portion 9 to a reservoirtank T. In addition, in that position, a further passage 33 connects theend of the head 27 remote from the seating 29 to the passage 32.

In the condition above we will designate the area of the piston 8 as A₁,the force in the spring 12 as S₁, the area of the seating 29 as A₂, theforce in the spring 28 as S₂, the area of the valve member 27 as A₃, andthe force in the spring 20 as S₃.

In the inoperative position shown in the drawing the piston 8 is heldagainst the stop member 15 by the spring 12, and the spool 10 is held inengagement with the piston 8 by the spring 20 in which position itcloses the passage 32 leading to the tank T.

To charge the system,, fluid under pressure from the pump P flowsthrough the spool 10 to the accumulator through the control chamber 24,the one-way valve 25 and the passage 19.

As the accumulator pressure rises the pump pressure will riseaccordingly but the pressure differential across the spool 10, caused bythe spring 20, will remain constant.

As the pressure in the accumulator approaches a cut-out point defined byS2/A2 the pilot dump valve 9 opens due to the fluid at that pressureacting on the valve head 27 over the area A₂ and the control chamber 24is exhausted to the tank T through the passage 31. This reduces the pumppressure immediately.

The accumulator pressure acting over A₁ then overcomes the force S₁ inthe spring 12 and urges the piston 8 and the spool 10 relatively awayfrom the stop member 15 until the spool 10 engages with the plug 11. Inthis position the pump P is in open unrestricted communications with thetank T through the passage 32, and the spool 10 isolates a passage fromthe bore portion 9 to the one-way valve 25 from the pump through thepassage 23 in the spool 10.

The force S₂ in the spring 28 acts to urge the valve member 27 againstits seating 29, once the outlet and pump pressures equalise, and fluiddisplaced by the area A₃ of the member 27 is passed through the seat 29and spool 10 to the passage 32.

The use of the services reduces the pressure in the accumulator 4. Whena cut-in pressure at a cut-in point is reached at S1/A1, the force S₁ inthe spring 12 is sufficient to return the piston 8 to its originalposition, and the spring 20 moves the spool 10 in the same direction topermit the accumulator to be charged again as described above.

In a typical example we arrange that

    S.sub.1 /A.sub.1 =1160 p.s.i. (80 bar);

    S.sub.2 /A.sub.2 =1450 p.s.i. (100 bar); and

    A.sub.3 =100 A.sub.2.

In a modification the piston 8 and the spool 10 can be integral witheach other or connected together against relative movement.

In the constructions described above the cut-in and cut-out pressurescan be altered charging the values of the springs 12 and 28. It isparticularly easy to change the spring 28 since it is readily accessibleafter removal of its external abutment. Alternatively the position ofthe external abutment can be altered to adjust the loading of the spring28.

I claim:
 1. An unloader valve assembly for vehicle hydraulic systems ofthe type incorporating an hydraulic high pressure pump, a reservoir andan hydraulic accumulator storing accumulator pressure for supplyingservices and adapted to be charged from said pump, said unloader valveassembly comprising a piston movable in first and second oppositedirections, a first spring biassing said piston in said first directionin opposition to accumulator pressure in said accumulator, a flowcontrol valve adapted to co-operate with said piston and movable infirst and second opposite directions, a second spring biassing said flowcontrol valve in said first direction corresponding to said firstdirection in which said piston is biassed by said first spring, meansdefining a control chamber supplied from said pump, pressure in saidcontrol chamber acting on said flow control valve in a directioncorresponding to the direction in which said flow control valve isbiassed by said second spring, and pump pressure acting on said flowcontrol valve in the opposite said second direction, and on said pistonin said first direction, an outlet for connection to said reservoir, apilot dump valve movable between an open position and a closed positionfor controlling communication between said control chamber and saidoutlet, and a third spring for biassing said dump valve into said closedposition to isolate said control chamber from said outlet, pressure insaid control chamber acting on said dump valve in opposition to saidthird spring to urge it into open position, and means for determining acut-out pressure for said accumulator comprising said third spring, andmeans for determining a cut-in pressure for said accumulator comprisingsaid first spring, said accumulator being charged by said pump throughpassage means including a substantially unrestricted passage in saidflow control valve, and said control chamber, said control chamber beingexposed to accumulator pressure during charging of said accumulator, andat said cut-out pressure said dump valve opens in response to saidcontrol pressure to connect said control chamber to said outlet, suchthat said flow control valve and said piston move against said first andsecond springs, said flow control valve being moved into an unloadingposition, in which it is maintained by said accumulator pressure actingon said piston, said dump valve subsequently closing, and followingreduction of said accumulator pressure due to operation of said servicesat said cut-in pressure, said first and second springs move said pistonand said flow control valve in said first direction to establishcommunication between said accumulator and said pump.
 2. An unloadervalve assembly as claimed in claim 1, wherein said third spring actssolely on said dump valve.